Method for controlling gap for sheet manufacturing measurement process

Abstract

To provide a method of controlling a gap for sheet manufacturing measurement processes.SOLUTION: A system for measuring a property of a continuous sheet comprises: a first scanner head 4 disposed on one side of the sheet, where the first scanner head 4 comprises a first sensor device 8 with a first movable sensor base 10 and a first adjustable actuator 12A, 12B configured to move the first sensor base along a first axis; a second scanner head 6 disposed on the opposite side of the sheet, where the first scanner head 4 and the second scanner head 6 define a sensor gap; and control means for controlling the first adjustable actuator, for measuring the distance of the sensor gap and maintaining the sensor gap distance at a desired gap distance.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates generally to a scanning measurement system for determining parameters of continuous sheet material during production, and more particularly to active measurement of sensor gap in a dual scanning head to control and maintain a constant sensor gap throughout the scan. [Background technology]

[0002] It is known to provide on-line measurements of properties of continuous sheet material during production, the purpose of which is generally to enable monitoring of the sheet manufacturing process with the intent of improving sheet quality while reducing the amount of substandard sheet material produced before undesirable process conditions are corrected.

[0003] Sensors for continuous flat sheet production processes typically use single-sided or double-sided packages with online sensors that traverse or scan the advancing web of sheet material during production. Scanning is usually performed in a transverse direction, i.e., perpendicular to the direction of sheet advancement. A typical transmission mode scanner system includes housings mounted on either side of the moving sheet (i.e., above and below the sheet) for movement across the sheet to measure selected sheet parameters. A channel or column may be formed between the upper and lower housings, with the sheet disposed within the channel. In operation of the scanning system, the housings synchronously advance across the sheet on tracks mounted on parallel horizontal frame members.

[0004] In practice, the housing functions as a platform for carrying a sensor that detects sheet characteristics, such as basis weight. Thus, for example, the upper housing can carry a radiation source, such as a nuclear beta radiation source, and the lower housing can carry a detector. In this case, the sensor can be used to perform basis weight measurements by measuring the intensity of radiation incident on the detector when a sheet is present compared to the beta radiation incident on the detector when no sheet is present. In other words, basis weight is measured by the beta radiation attenuated by the sheet material. Online measurements during continuous sheet material production are difficult to perform accurately. Scanning sensor systems are susceptible to various errors. For example, if the vertical distance separating the housings is not kept constant during scanning, the incident radiation intensity will change independently of variations in sheet characteristics.

[0005] A reflective mode scanner system includes a sensor mounted in an upper housing facing the moving sheet. In operation, radiation from a radiation source within the upper housing is directed toward the sheet, and reflected radiation is received by a detector within the upper housing. A distance between the sensor and the moving sheet must be maintained.

[0006] Various techniques have been used to correct for sensor position errors such as misalignment. However, current methods do not provide for dynamic active measurement and control of the sensor gap within the dual housing of a scanning system. Summary of the Invention

[0007] The present invention relates to a technique for controlling the sensor gap in the measurement channel between dual scanner heads during a transmission measurement mode of operation. Many sensors used in sheet manufacturing environments are sensitive to the gap between the scanner heads. For example, a metric basis weight sensor effectively measures both the weight of the product and the weight of the air in the column between the sensor halves, which is a function of the gap distance and the air temperature. Other types of sensors use a subtraction method to measure the distance to the product's surface and subtract this from the total gap to obtain a caliper measurement, such as in a laser caliper sensor. By controlling the gap itself, correction for the effect can be simplified for each sensor.

[0008] The present invention also relates to a technique for controlling the measurement gap between a sensor head and a corresponding reflective working surface of a mounting head or frame in a reflective scanner system.

[0009] The present invention is based in part on the recognition that some sensors can benefit from closed loop control in which an actuator is used to maintain a constant sensor gap throughout a scan by closing an error signal on the gap measurement sensor. In one aspect, the present invention provides a system for measuring a property of a continuous sheet having a first side and a second side and progressing in a downstream machine direction, comprising: (a) a first scanner head disposed adjacent a first side of the sheet, (i) a first sensor device having a first movable sensor base defining a first operating surface facing a first side of the seat; (ii) a first scanner head comprising: a first adjustable actuator configured to move a first sensor base along a first axis; (b) a second scanner head disposed adjacent to a second side of the sheet, the second scanner head having a second operative surface facing the second side of the sheet, the first operative surface and the second operative surface defining a sensor gap through which the continuous sheet advances; (c) means for measuring a first sensor gap distance and generating a first signal representative of the measured sensor gap distance; (d) control means for controlling the first adjustable actuator in response to the first signal to maintain the sensor gap distance at a desired gap distance.

[0010] In another aspect, the present invention provides a system for monitoring a characteristic of a continuous sheet having a first side and a second side and traveling in a downstream machine direction, the system comprising: (a) a first application head disposed adjacent to a first outer surface of the sheet, (i) a first movable sensor base defining a first operating surface facing a first exterior surface of the seat; (ii) a first actuator configured to move the first sensor base along a first axis perpendicular to the first working surface; (b) a second placement head disposed adjacent to the second outer surface of the sheet, the second placement head having a second operating surface facing the second outer surface of the sheet, the first operating surface and the second operating surface defining a sensor gap through which the continuous sheet advances, the first placement head and the second placement head being aligned and configured to move synchronously in a transverse direction perpendicular to the machine direction, and at least one sensor device being mounted to the first placement head, the second placement head, or both; (c) means for measuring the sensor gap distance and generating a signal representative of the measured sensor gap distance; (d) control means for controlling the first actuator in response to the signal to maintain the measured gap distance at the desired gap distance.

[0011] In a further aspect, the present invention is directed to a method of controlling a system for monitoring a characteristic of a continuous, traveling web, the system including a sensor head having a working surface connected to an actuator for moving the working surface, the method comprising adjusting a sensor gap between the working surface and the traveling web, (a) monitoring the distance between a working surface and a traveling web and generating a distance signal indicative of the measured sensor gap distance; (b) comparing the measured sensor gap distance with a target sensor gap distance; (c) manipulating the working surface to adjust the sensor gap distance. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 2 is a cross-sectional side view of a dual scanner head. [Figure 2] FIG. 1 is a cross-sectional side view of a dual scanner head operating in a transmissive mode. [Figure 3] FIG. 1 is a cross-sectional side view of a dual scanner head operating in a reflective mode. [Figure 4] denotes a scanner in a web manufacturing or converting system. DETAILED DESCRIPTION OF THE INVENTION

[0013] FIG. 1 shows a dual scanning system 2 having scanner heads 4 and 6. Upper scanner head 4 houses a sensor device 8 supported on a base plate 10 having a planar operating surface 11. Actuators 12A and 12B are attached to a lower surface 24 of upper scanner head 4 and are fixed to base plate 10. Upper sensor head 4 also houses a sensor device 28 supported on a base plate 30. Actuators 32A and 32B are attached to a lower surface 24 of upper scanner head 4 and are fixed to base plate 30 having a planar operating surface 31. Upper scanner head 4 includes upper sheet deflectors 50 and 52.

[0014] The upper scanner head 6 contains a sensor device 14 connected to a base plate 16 having a planar operating surface 17. Actuators 18A and 18B are attached to an upper surface 26 of the lower sensor head 6 and are fixed to the base plate 16. The lower scanner head 6 also contains a sensor device 34 connected to a base plate 36 having a planar operating surface 37. Actuators 38A and 38B are attached to the upper surface 26 of the lower scanner head 6 and are fixed to the base plate 36. The lower scanner head 6 includes lower sheet deflectors 54 and 56.

[0015] The displacement between the operating surface 11 of the sensor device 8 and the operating surface 17 of the sensor device 14 defines a gap 60, a distance that can be measured by a displacement measurement means. A suitable device is an inductive sensor having an RF or z-coil 20 disposed within the sensor device 8 and measuring the distance from the z-coil 20 to a reference surface or plate 22, which is the sensor base plate 16. Inductive sensors are particularly suitable when the web or sheet being monitored is paper or plastic. The distance of the gap 60 can also be measured by optical displacement, laser-based triangulation, inductive sensors, radar, capacitive sensors, magnetic field-based sensors, microwave radiation-based sensors, and acoustic-based sensors.

[0016] Similarly, the displacement between the operating surface 31 of the sensor device 28 and the operating surface 37 of the sensor device 34 defines a gap 62, the distance of which can be measured by a displacement measuring means. When the sheet product includes a metal substrate, a preferred displacement measuring mechanism includes an electromagnetic coil 40 disposed within the sensor device 28 and first and second magnetic sensors 42 and 44 disposed within the sensor device 34. The two magnetic sensors are preferably in tandem and aligned axially with the coil. The electromagnetic coil 40 is connected to a DC or AC source (not shown) to generate a magnetic field that is measured by a pair of magnetic sensors, such as fluxgate magnetic sensors. Instead of using an electromagnetic coil and associated drive current, a permanent magnet can be used as the source of the magnetic field. This magnetic displacement sensor reads through a metal foil material, such as a coated metal electrode, and is described in U.S. Patent No. 11,519,710 to Tixier et al., incorporated herein by reference.

[0017] Each of the actuators can include a multilayer piezoelectric actuator in the form of a piezoelectric stack, which can be used in conjunction with leverage to increase actuation distance at the expense of peak force. The piezoelectric actuator can expand or contract upon actuation in response to an applied electric field. Thus, in one application, when the stack is energized, it extends, pushing the base plate to which it is fixed in a vertical direction perpendicular to the associated actuation surface. As shown in FIG. 1, each of the scanner heads 4 and 6 can include multiple sensor devices, each defining a sensor gap 60, 62. The displacement distances of the gaps can be the same or different, and each gap is controlled independently of the others.

[0018] 2 illustrates a dual scanning system 72 in which an upper scanner head 74 houses a sensor device 78 supported on a base plate 80 having a planar operating surface 81. Actuators 82A and 82B are attached to a lower surface 94 of the upper scanner head 74 and are fixed to the base plate 80. The sensor device 78 includes a radiation source 100. The lower scanner head 76 houses a sensor device 84, which includes a radiation detector 102 connected to a base plate 86 having a planar operating surface 87. Actuators 88A and 88B are attached to an upper surface 96 of the lower sensor head 76 and are fixed to the base plate 86. Displacement between the sensor device 78 and the sensor device 84 is measured using a z-coil 90 and a reference surface or plate 92.

[0019] A continuous web or sheet of material 130, such as paper, plastic, or fabric, advances in the machine direction (MD) through a measurement channel between the upper and lower scanner heads. In one embodiment, radiation source 100 includes a nuclear beta radiation source for measuring properties such as basis weight of material 130 by measuring the intensity of radiation incident on detector 102 when material 130 is present compared to the beta radiation incident on the detector when no material is present. Readings are susceptible to error when the vertical distance separating radiation source 100 and radiation detector 102 varies. Depending on the material being monitored, other radiation sources, such as X-rays and infrared radiation, may also be used.

[0020] In the present invention, a PID controller 104 receives signals from gap measurement means 90 and 92, and the controller is configured to send gap modulation signals to voltage sources 106, 108, 110 and 112. Increasing amplitudes, decreasing amplitudes, and both polarities of the voltage sources all cause expansion or contraction.

[0021] In the scanning system 72 of Figure 2, both the upper sensor device 78 and the lower sensor device 84 are equipped with actuators for adjusting the gap distance. As will be apparent, the gap displacement can be modulated by configuring only one of the sensor devices with one or more actuators. However, by having both sensor device 78 and sensor device 84 operable with actuators, a faster response is achieved.

[0022] It should be noted that in the present invention, it is not the vertical position of either scanner head 74 or scanner head 76 that is adjusted by the actuator. Scanning heads (or mounting heads) are typically relatively large structures housing sensor devices and other components. Sensor devices, such as radiation sources and detectors, are subject to mechanical vibrations, typically in the 10-100 Hz range and primarily limited to 1 kHz or less. The present invention compensates for the adverse effects caused by vibrations by actuating the smallest mass in each sensor. The mass that must be controlled is the smallest mass that affects the sensing principle used. For example, the distance from the radiation source to the web affects the intensity and therefore the measurement, whereas some circuitry does not affect the measurement, so the sensor electronics do not need to be controlled, but the radiation source does. The speed of the feedback mechanism must be able to offset these vibrations. This can be achieved with piezoelectric actuators. The response time of these actuators can be up to two orders of magnitude faster than this (1 microsecond) for actuation (gap) distances of tens to hundreds of micrometers.

[0023] 3 shows a scanning system 102 in which an upper scanner (or mounting) head 104 houses a sensor device 108 supported on a base plate 110. Actuators 112A and 112B, attached to the lower surface of the upper sensor head 104, are fixed to the base plate 110, which has a planar operating surface 111. The actuators move the base plate 110 vertically, with their axes perpendicular to the planar operating surface 111. The sensor device 108 includes a radiation emitter source 154 and a receiver detector 152.

[0024] A continuous web or sheet of material 150 moves past an upper scanner head in MD. The displacement between working surface 111 and the upper surface of material 150 is measured by laser triangulation device 120. In one embodiment, radiation source 154 includes an infrared radiation source that is focused on the sheet of material 150. Radiation reflected from the material is captured by radiation detector 152. Actuators 112A and 112B maintain the gap distance between working surface 111 and the upper surface of material 150.

[0025] FIG. 4 illustrates a dual scanner 170 in a web manufacturing or converting system. The scanner 170 travels back and forth across a moving web (not shown) in a transverse direction perpendicular to the MD direction. The scanner 170 is driven by a motorized drive mechanism (not shown). The scanner 170 includes an upper mounting head 180 and a lower mounting head 182 mounted on carriages supported by upper and lower rails 172 and 174, respectively. The movement of the two aligned mounting heads is synchronized, and the heads define a channel or gap 184 through which the web travels. Support structures 176 and 178 maintain the separation of the rails. The web (not shown) passes horizontally between the support structures 176 and 178 as the scanner travels over at least one surface of the web. In this embodiment, the scanner 170 has sensor devices 186 and 196 operating in a transmission mode to measure one or more characteristics of the web.

[0026] The foregoing has described the principles, preferred embodiments, and modes of operation of the present invention. However, the invention should not be construed as limited to the particular embodiments discussed. Accordingly, the above-described embodiments are to be considered illustrative rather than restrictive, and it will be understood that changes can be made therein by those skilled in the art without departing from the scope of the invention as defined by the following claims.

Claims

1. 1. A system for measuring a property of a continuous sheet having a first side and a second side and traveling in a downstream machine direction, comprising: (a) a first scanner head disposed adjacent a first side of the sheet, (i) a first sheet deflector extending along the machine direction immediately adjacent the first side of the sheet; (ii) a first sensor device having a first movable sensor base defining a first operational surface facing the first side of the seat; (iii) a first scanner head comprising: a first adjustable actuator disposed between the first sheet deflector and the first movable sensor base, the first adjustable actuator configured to move the first movable sensor base along a first axis; (b) a second scanner head disposed adjacent the second side of the sheet, the second scanner head including a second sheet deflector extending along the machine direction immediately adjacent the second side of the sheet and a second operative surface facing the second side of the sheet, the first operative surface and the second operative surface defining a sensor gap through which the continuous sheet advances; (c) means for measuring the sensor gap distance and generating a first signal representative of the measured sensor gap distance; (d) control means for controlling the first adjustable actuator in response to the first signal to maintain the sensor gap distance at a desired gap distance.

2. 1. A system for monitoring a characteristic of a continuous sheet having a first side and a second side and traveling in a downstream machine direction, comprising: (a) a first placement head disposed adjacent a first outer surface of the sheet, (i) a first sheet deflector extending along the machine direction immediately adjacent the first outer surface of the sheet; (ii) a first movable sensor base defining a first operating surface facing the first exterior surface of the seat; (iii) a first actuator disposed between the first sheet deflector and the first movable sensor base, the first actuator configured to move the first movable sensor base along a first axis perpendicular to the first operating surface; (b) a second placement head disposed adjacent to a second outer surface of the sheet, the second placement head comprising: a second sheet deflector extending along the machine direction immediately adjacent to the second outer surface of the sheet; and a second operative surface facing the second outer surface of the sheet, the first operative surface and the second operative surface defining a sensor gap through which the continuous sheet passes, the first placement head and the second placement head being aligned and configured to move synchronously in a transverse direction perpendicular to the machine direction, and at least one sensor device being mounted to the first placement head, the second placement head, or both; (c) means for measuring the sensor gap distance and generating a signal representative of the measured sensor gap distance; (d) control means for controlling the first actuator in response to the signal to maintain the measured sensor gap distance at a desired gap distance.

3. 3. The system of claim 2, wherein the second mounting head comprises a second movable sensor base defining the second operating surface facing the second outer surface of the sheet, and a second actuator configured to move the second movable sensor base along a second axis, and the control means is configured to control the first actuator and the second actuator in response to the signal to maintain the measured sensor gap distance at the desired gap distance.

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